Why Stateless EJBs are pooled?

Question

What is the reason why the Application Servers pool the Stateless EJBs?

I can understand that it is usefull to control the workload of the application for incomming invokations, but this only justifies the pooling of the EJBs that server as FAÇADE with the invoker client.

Does it have any benefit to pool the internal EJBs (those that are not exposed and only invoked internally to perform business logic)?? instead of use a shared single instance (like Spring does).

I can think about at least one downside: a highly used internal EJB could act as a bottleneck.

Answer 1

Stateless session bean EJBs are not necessarily thread-safe. They can be holding resources like JMS sessions which cannot be shared with more than one thread at a time so the server will pool them so that it can serve multiple requests for the same bean concurrently (JMS resources are also pooled, but I'm just using that for the sake of example).

Answer 2

I would also like to know why stateless EJBs are pooled. But i want to know why they're pooled rather than being created and destroyed on demand. The fact that instances can be reused for unrelated requests significantly complicates the implementation of stateless beans (it means you have to be incredibly careful about the use of instance fields), and i don't see any significant benefit to it.

Specifically, i don't see any performance benefit to it. I poked through the implementation of stateless beans in JBoss (6, IIRC), and its only the bean instance itself that's pooled; the plumbing to handle method invocation is recreated from scratch each time it's used. That means that the only performance saving is a single object creation, which should be a trivial amount of time. The only situation in which i can see it being non-trivial is if the bean acquires heavyweight resources, and holds on to them between invocations. However, in that case, the bean is really being used as a glorified, badly-managed, pool; the correct solution would be to pool the resource directly!

Now, EJB has been around a long time. Back when they first came out, object creation was expensive, so it made sense to pool them. But those days are long gone. Why was pooling not dropped in EJB3?